1. Field of the Invention
The present invention relates to energy-trapping piezoelectric resonators which vibrate in a thickness-extension vibration mode, and more particularly, to an energy-trapping piezoelectric resonator which vibrates in a thickness-extension vibration mode and is incorporated in a piezoelectric filter or a piezoelectric oscillator.
2. Description of the Related Art
An example of an energy-trapping piezoelectric resonator vibrating in a thickness-extension mode is disclosed in Japanese Examined Patent Publication No. 7-70941. The structure of this resonator will now be described with reference to FIG. 20. A piezoelectric resonator generally designated by 51 includes a rectangular piezoelectric substrate 52 polarized in a thickness direction thereof. A resonance electrode 53a is disposed on the central portion of the upper surface of the substrate 52 and a resonance electrode 53b is disposed at the central portion of the lower surface of the substrate 52 so as to opposedly face the resonance electrode 53a. The resonance electrodes 53a and 53b are electrically connected to lead electrodes 54a and 54b, respectively, which are provided along opposite edges of the substrate 52.
The above-described piezoelectric resonator 51 experiences the following problem. The resonance electrodes 53a and 53b must be located at the central portions of the substrate 52 so as to trap vibration energy in the opposing region (resonance portion) located between the resonance electrodes 53a and 53b. Accordingly, a comparatively large piezoelectric substrate 52 is required to ensure a sufficient area around the resonance portion of the substrate 52. This prevents the size of the piezoelectric resonator 51 from being reduced.
To overcome the above drawback, Japanese Unexamined Patent Publication No. 2-235422 discloses an energy-trapping piezoelectric resonator utilizing the thickness-extension vibration mode which does not require a large area around the resonance portion of the piezoelectric substrate. In this resonator 61, as illustrated in FIG. 21, a resonance electrode 63a is disposed on the upper surface of a narrow piezoelectric ceramic substrate 62 and a resonance electrode 63b is disposed on the lower surface of the ceramic substrate 62. The resonance electrodes 63a and 63b are arranged so as to cover the overall width of the substrate 62. Also, a resonance portion is provided in an opposing region between the resonance electrodes 63a and 63b at the center of the substrate 62 in the longitudinal direction. The resonance electrodes 63a and 63b extend to lateral edges 62a and 62b, respectively, in the widthwise direction of the substrate 62.
In the above-described piezoelectric resonator 61 in which the thickness-extension vibration mode is generated in the narrow piezoelectric substrate 62, unwanted vibrations are caused due to the relationship between a width W and a thickness T of the substrate 62. Thus, Japanese Unexamined Patent Publication No. 2-235442 states that unwanted spurious responses in a range between the resonant frequency and the anti-resonant frequency can be reduced by determining the ratio W/T in the following manner. The ratio W/T is determined to be approximately 5.33 at a resonant frequency of 16 MHz when a fundamental wave is utilized. On the other hand, W/T is determined to be approximately 2.87 at a resonant frequency of 16 MHz when a third-order wave is utilized.
When a prototype of the piezoelectric resonator 61 was actually prepared and tested, however, unwanted spurious responses were still generated between the resonant frequency and the anti-resonant frequency, thereby preventing the filter from having good resonance characteristics.